Intelligent switch for connecting power to a load

ABSTRACT

A switching circuit includes a microchip which, in response to a signal from a signal switch, controls the operation of a power switch which, when closed, connects a load to a battery. The microchip can monitor the status of the battery and control the power switch to ensure optimum operation of the load and optimum usage of the energy in the battery. The microchip can control the connection of the load to the battery in different ways according to the manner of operation of the signal switch.

[0001] This invention relates generally to an intelligent switch whichis suitable for controlling the use of a battery driven device such as aflashlight, toy, motor or the like and, more particularly, is concernedwith an intelligent electrical device of the kind described in theapplicant's international application No PCT/ZA99/00107.

BACKGROUND OF THE INVENTION

[0002] In the PCT application mechanical switches function as aman-machine-interface (“MMI”) between the device and an operator. TheMMI functions are controlled by very low current signals using touchpads, carbon coated membrane type switches or similar mechanisms. Amicrochip is responsive to input signals from the MMI and can, accordingto application, be used in various ways. For example with a flashlightthe MMI may control the on/off operation of the flashlight, cause theflashlight to be turned off after a predetermined time, provide anindication of battery strength, or enable the flashlight to be operatedwith a desired flashing sequence. Various other features can also beachieved through the judicious use of the microchip and reference ismade to the specification of the PCT application for a furtherdescription of such features.

[0003] The applicant is aware of a number of systems which provide anindication of the charge left in a battery.

[0004] Osterhout et al (U.S. Pat. No. 4,876,632) describes a circuitwhich detects the closed circuit voltage of a battery pack used in aflashlight. Based on predetermined reference voltages the battery lifeis shown in one of three categories. The circuit is activated by anon/off switch of a flashlight and does not function when the flashlightis off. This is to ensure that there is no power consumption when theflashlight is off. If however the circuit should function when the lightis off the open circuit voltage of the battery will be measured and thiswill give a misleading indication of the available battery life.

[0005] Mallory (U.S. Pat. No. 4,499,525) and Weber (U.S. Pat. No.5,821,697) relate to the provision of constant illumination byflashlights. Much can be gained in terms of quality of light andoperating life of light bulbs by maintaining constant power through alight bulb. It is to be noted that in most instances the voltage whichis supplied by a battery varies quite extensively over its usable life.Mallory makes use of an RC filter to determine an effective current andtime measurement. Based on component selection the duty cycle of acurrent delivered to a light bulb at a specific voltage can bedetermined.

[0006] Weber uses a different technique wherein power is dissipated in avariable resistor or transistor element. This approach is wasteful ofpower.

SUMMARY OF THE INVENTION

[0007] The present invention is concerned with various modifications andimprovements which can be made to the microchip and to the embodimentsof the invention described or claimed in the specification of the PCTapplication. To the extent which may be necessary for an understandingof the present invention the disclosures in the specification of the PCTapplication are to be read in conjunction with this specification and,where applicable, such disclosures are to be deemed to be incorporatedinto this specification.

[0008] According to a first aspect of the invention there is provided aswitching circuit for controlling the supply of power from anexhaustible power source to a load, which includes a control circuit, apower switch which is controlled by the control circuit and which, whenclosed, connects the power source to the load, and at least one signalswitch which is connected to the control circuit, and wherein thecontrol circuit provides at least one function selected from thefollowing:

[0009] (a) the control circuit causes the power switch to be operated ina manner which is dependent on the operation of the signal switch,

[0010] (b) the control circuit compares the voltage of the power source,when the power source is connected to the load, to at least onereference voltage thereby to provide an indication of the status of thepower source,

[0011] (c) the control circuit monitors the voltage of the power sourceand when the power source voltage drops below the operating voltage ofthe control circuit, the power switch is controlled by the signal switchto connect the power source to the load,

[0012] (d) the control circuit monitors the voltage of the power sourceand the power switch is latched on or off, according to requirement,when the power source voltage drops below the operating voltage of thecontrol circuit, and

[0013] (e) the control circuit controls the duty cycle of the powerswitch, in a manner which is dependent on the voltage of the powersource, to provide a substantially constant supply of power by the powersource to the load.

[0014] A specific sequence of operations of the signal switch may beinterpreted to provide for the application of power to the load for anindefinite period, ie. effectively permanently, until terminatedaccording to a different criterion.

[0015] In another embodiment if the signal switch is held on for anextended period the power switch may be operated in such a way that adimming or reduced power operation results.

[0016] In a variation the power switch is closed for a predeterminedperiod which is a function of the duration of a time period for whichthe signal switch is operated.

[0017] According to a second aspect of the invention the control circuitincludes the ability to monitor the voltage level of the exhaustiblepower source (ie. a battery) which is connected to the load and when thevoltage level drops below the operating voltage of the control circuitan input signal from an input switch to the control circuit is useddirectly to control the application of power from the battery to theload. It follows that, although the control circuit will be disabled andwill not exhibit all its design functions when the battery voltage istoo low, the control circuit is nonetheless capable of allowing themaximum extraction of energy from the battery, when required. This canbe done without voltage sensitive parts (eg. oscillator and control ordecision making logic) of the circuit being operational.

[0018] The switching circuit may include a comparison unit for comparingthe voltage of the power source, when the power source is connected tothe load, to at least one reference voltage thereby to provide anindication of the status of the power source.

[0019] The voltage which is compared to the reference voltage is thusthe closed circuit voltage of the power source.

[0020] The switching circuit may include at least one light emittingdevice which is energised to provide a visual indication of the saidpower source status.

[0021] The switching circuit may include a memory unit for storing ameasure of the voltage of the power source, particularly the closedcircuit voltage. This facility means that an indication of the status ofthe power source is available even though the power switch is open.

[0022] The switching circuit may include a measuring unit for measuringthe open circuit voltage of the power source, ie. when the power switchis open, and the comparison unit may be adapted for comparing the opencircuit voltage measurement to the closed circuit voltage of the powersource, and for providing a signal if the open circuit voltage dropsbelow the previously measured closed circuit voltage.

[0023] The said reference voltage may be stored in the memory unit.Preferably two or more reference voltages are stored in the memory unit.

[0024] The control circuit, in response to the closed circuit voltage ofthe power source, may control the power switch in order to vary the dutycycle of the current which is passed to the load to achieve asubstantially constant power supply to the load.

[0025] Preferably in the case in which light emitting devices are usedto provide a visual indication of the said power source status at leastone of the said light emitting devices is used to act as afind-in-the-dark indicator by causing the said light emitting device toflash at a very low duty cycle, at the same time continuously showingthe battery status.

[0026] After the circuit has been powered up or down the memory unit maybe reset with a fresh measurement of the prevailing open circuit voltageor of the closed circuit voltage of the power source. The controlcircuit may be adapted to close the power switch automatically and for ashort period of time for obtaining a closed circuit voltage measurementof the power source. This can be done for example once every 24 hours.

[0027] According to a variation of the invention the power switch islatched on or off, according to requirement, when the power sourcevoltage is too low to cause normal operation of the control circuit ie.when the control circuit enters a non-functioning or reset state whichis dependent on the battery voltage.

[0028] According to a different aspect of the invention if the switchingcircuit includes a first signal switch for on/off selection and a secondsignal switch which selects a plurality of functions then the secondsignal switch may be enabled to provide an “off” command to the controlcircuit, and hence of the load which is connected to the controlcircuit, if the second signal switch is activated a predetermined period(eg. 2 seconds) after the last operation of the second signal switch orfirst signal switch.

[0029] According to a further aspect of the invention the controlcircuit may include a voltage regulation capability which provides aregulated voltage through the power switch to the load.

[0030] The switching circuit may include a timer which controls the saidpredetermined period and wherein the timer is reset each time the signalswitch is operated.

[0031] The control circuit may include an input pin or contact to whichan input signal is applied from the signal switch. This may take placein any appropriate way eg. by activating the signal switch which isconnected to the input pin or contact. Alternatively the control circuitmay detect activity by a user in any other appropriate way eg. bymonitoring activities at or to all input terminals to the controlcircuit and, each time an activity is detected, resetting the counter ortimer. Thus the delayed switch-off function will only occur if noactivity is detected for the full duration of a predetermined period(prior to the switch-off function).

[0032] The switching circuit may include a first signal switch which isconnected to an edge triggered input of the control circuit and a secondsignal switch which is connected to a state triggered input of thecontrol circuit and wherein a signal input by the first signal switchcauses the power switch to change from a state previously determined bya signal input from the second signal switch, and a signal input by thesecond signal switch does not cause the power switch to change from astate previously determined by a signal input from the first signalswitch.

[0033] In this specification the phrases “control circuit” and“microchip” are used interchangeably.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention is further described by way of examples withreference to the accompanying drawings in which:

[0035]FIG. 1 is a block diagram illustrating the use of a microchip forcontrolling an electrical load in accordance with the principles of theinvention,

[0036]FIG. 2 illustrates typical components of the microchip,

[0037]FIG. 3 is a simplified flow chart of a sequence of operationswhich can arise during the operation of the microchip,

[0038]FIG. 4 is a flow chart, similar to FIG. 3, of a slightly differentsequence of operations,

[0039]FIG. 5 is a flow chart illustrating operation of the microchipunder a different set of conditions,

[0040]FIG. 6 illustrates a variation of the invention,

[0041]FIG. 7 is a block diagram representation of a circuit according tothe invention for controlling the supply of power from a battery powersource to a load which, in this case, is a lamp,

[0042]FIG. 8 is a more detailed representation of an integrated circuit(microchip) used in the circuit of FIG. 7,

[0043]FIG. 9 is a representation of a typical modulated signal which isdelivered by the circuit of FIG. 8 to the load,

[0044]FIG. 10 is a flow diagram representation of certain steps in theoperation of the circuit of FIG. 8,

[0045]FIG. 11 is a graphical representation of a duty cycle as afunction of voltage for a 3.6V bulb,

[0046]FIG. 12 shows voltage error and duty cycle curves respectively asa function of power source voltage,

[0047]FIG. 13 illustrates a variation of the invention, and

[0048]FIG. 14 shows a way of packaging the circuit of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0049]FIG. 1 of the accompanying drawings schematically illustrates aswitching circuit 10 which includes a microchip or control circuit 12, abattery 14, a load 16, and a signal switch 18.

[0050] The arrangement shown in FIG. 1 is similar to what has beendescribed in the specification of international application No.PCT/ZA99/00107 and, for a detailed description of the operation of thearrangement, reference is made to the specification of the internationalapplication.

[0051] The switch 18 functions as an interface between an operator orany appropriate actuating mechanism, and the control circuit 12, andhence is referred to as a man-machine-interface (MMI) This term ishowever adopted merely for the sake of convenience for, as noted, theswitch could be operated by human intervention or by any mechanism eg. aclosing door or other device which acts on the switch. The controlcircuit 12, in response to signals input from the MMI, causes power tobe applied from the battery 14 to the load 16. The load may varyaccording to application and for example may be a light bulb, a heateror the like.

[0052]FIG. 2 illustrates internal components of the control circuit 12.Voltage from the battery 14 is applied to terminals 22 and the switch 18controls the application of power to a control circuit or unit 30 insidethe circuit 12. The circuit 12 additionally includes at least a timer32, a power or load switch 34 and, optionally, a voltage regulator 36.

[0053] In general terms the control unit 30, in response to a signalfrom the switch 18, causes operation of the power switch 34 and therebyconnects the battery 14 to the load 16, or disconnects the battery fromthe load. The power switch 34 may or may not be part of a combinedintegrated circuit with the signal switch.

[0054] The switch 18 may vary in its construction and for example, maybe a push button switch ie. of momentary actuation, or a slide switchie. on or off (in two stable states). If the control circuit 12 isfabricated separately from the switch then, in advance, the nature ofthe switch with which the control circuit 12 is to be used may not beknown. FIGS. 3 and 4 illustrate different flow charts which arisethrough different phases of operation of the microchip of the inventionwith different respective switch types.

[0055] Assume that the control switch 18 is a push button, ie. apush-to-make and release-to-break, switch. Referring to FIG. 3, from anoff state 40, if the switch is operated to make contact for less than apredetermined time interval T, say less than 0.5 seconds (step 42), thenthe switch is regarded as a push-to-make switch and the power is latchedon (step 44). To switch the power off (step 46) contact must again bemade or, alternatively, switching off can be initiated by a function ofthe microchip eg. auto shut-off. These aspects are indicated by means ofa block 48 labelled “external”.

[0056]FIG. 4 illustrates a sequence of operations for a slide switch,ie. for push-to-make and push-to-break operation. In this case, from anoff state 40, if contact is made for more than a period T of, say, 0.5seconds, (block 42A), the switch is regarded as a slide switch and poweris kept on (44) until turned off (46) by means of an external mechanism48, eg. auto shutoff or operation of the switch.

[0057] The aforementioned type of operation, with both switch types, ishighly desirable under certain conditions for example in the case of aflashlight which is hand held by a law enforcement officer. Typicallytwo distinct functions are required. The first is to have the lightswitch on while scanning an area. In this case currently availableproducts provide a push-to-make type of button and the light is on whilethe button is pushed and goes off immediately upon release. Clearly onehand is occupied full-time to have the light on.

[0058] Secondly, to switch the light on for an extended period of time,a twisting operation is required. The front or back end of theflashlight is, in some cases, turned to have a slide-switch action. Thisnormally requires the use of two hands which may not be convenient.

[0059] The present example of the invention provides an elegant solutionto this type of situation. A momentary press on the switch causes theflashlight to be turned on for an extended period. Thus single handoperation can be resorted to. The light shines until the button is againpushed or auto shut-off occurs. This can be very convenient for anofficer holding a gun in one hand and wanting to switch his flashlighton for an extended period of time.

[0060] Permanent-on selection can easily be accommodated. In this casean override of the auto-shut off procedure is needed. This can beprovided by stipulating that a on-off-on action within a short period,say within two seconds, is interpreted by the control unit 30 as apermanent-on mode selection. In other words three presses on a pushbutton or three slides of a slide switch will be interpreted as beingpermanently on. The sequences are indicated in FIGS. 3 and 4respectively by the steps 42, 50 and 52 and 42A, 50A and 52Arespectively, resulting in each case in a permanently on state 54.

[0061] The mode selected is indicated for example by means of a visualdisplay device such as a light emitting diode 56. The LED may also beused as a find-in-the-dark facility. For example a single flash everytwo seconds on this indicator can indicate an on state with auto shutdown in operation. Two flashes in quick succession with a delay ofapproximately two seconds before the next two flashes may indicate apermanent on mode. More flashes in quick succession with a delay beforethe next flashes may indicate other selected modes.

[0062] An important aspect of the invention is operation in a lowvoltage region. A flashlight or toy (ie. the load 16) operating from thebattery 14, will encounter a situation in which the battery is depleted.Often the battery runs down gradually with its voltage level slowlydropping. Typically the voltage may be within the operationalspecification of the microchip or control circuit 12 and then it willslowly deplete and drift out of specification.

[0063] It is desirable, under certain conditions, to give the user asmuch use as possible from a set of batteries and also to try and matchthe performance of a mechanical switch which is not normally voltagedependent or responsive.

[0064] The microchip 12 is, in accordance with an aspect of theinvention, supplied with a reset circuit 60 (see FIG. 2) which functionsin a manner which is known in the art. Below a certain voltage thresholdthe reset circuit 60 keeps the microchip 12 in a reset ornon-functioning state. Above the threshold voltage the reset circuit 60allows the control circuit 12 to operate in accordance with its designrequirements.

[0065] In the present case a reset signal, produced by the circuit 60,is used to connect the signal from the switch 18, ie. from the MMI,directly to the power switch 34 when the circuit is in a reset state. Insuch a state the power switch conducts current when the MMI switch isclosed.

[0066] The effect of this is that the auto shut off and other functionsare disabled when the voltage of the battery 14 is below designspecification. If a push-to-make switch is used to select the on statewith a single short push, the power is turned off when the reset circuitturns on. In a flashlight example however the user is able to shine thelight, although it is very dim, by keeping the push button depressed.

[0067] In the case of a slide switch the auto shut off and otherfunctions are again disabled at the time the reset circuit turns on but,with the flashlight in an on state, the flashlight stays on as thebattery 12 gets fully depleted. The flashlight will switch off once theslide switch is moved to a non-contact position. The find-in-the-darkand similar functions are disabled since the relevant portion of themicrochip is not operational when the reset circuit 60 is turned on.

[0068] In other embodiments the power switch 34 may be latched on/offwhen a reset state is entered. Latching on may be important in caseswhen continuous operation to the lowest possible voltage is required eg.with a flashlight. An off selection may be important in a case where avoltage which is too low may damage the product eg. with an electricmotor.

[0069]FIG. 5 illustrates, in flow chart form, the aforementionedsequence of operations. The voltage 70 of the battery 14 is monitored(step 72). If the voltage is within specification then the controlcircuit 12 functions normally (step 74). If the voltage drops belowspecification then the reset circuit 60 detects this (step 76) and thepower switch 34 is then made directly responsive to the MMI switch 18(step 78). Depending on requirement and in particular on the nature ofthe load 16 the power switch 34 can be latched on or off (step 80)

[0070] Assume that the control circuit 12 is responsive to the switch 18and to a second switch 18A shown in dotted lines in FIG. 1. In otherwords the MMI has two signal switches. The first switch may for examplemay be used for on/off selection while the second switch may be used toselect the mode of operation and, when pressed, may step sequentiallythrough various modes such as a first level of dimming, a second levelof dimming, fast flash, slow flash, etc, in a cyclical manner.

[0071] A user may get confused with the two switches. Assume for examplethat a dimming mode is selected and operated for a period of time. Ifthe user wants to switch the flashlight off but inadvertently pressesthe mode switch 18A instead of the first switch 18 the flashlight againstarts flashing. This may be bothersome and confusing for some users.Under these conditions the control unit 30 may detect operation of themode selection switch 18A, when operated after a predetermined periodof, say, 10 seconds, and change its function so that such operationresults in an off function ie. the load 16 is disconnected from thebattery 14.

[0072] In another embodiment of the invention the control circuit 12includes a voltage regulator 36 This is used to provide a regulatedvoltage, derived from the battery 14, to the load 16. This may be highlydesirable under certain conditions for it can be used to prevent anovershoot of the voltage, applied to the load, when the switch 34 isturned on and can enable much longer operation at an optimal voltage.

[0073] Assume for example that a flashlight operating from a 6V batterypack has an output from the current switch regulated to 4.5V. As thebattery gets depleted from its initial 6V the light will have optimaland constant performance until the battery voltage drops to 4.5V. Whenthe battery voltage drops below this level the current supply to theload will gradually diminish and will continue to diminish until thebattery is totally exhausted. This feature helps in the design andchoice of bulb for use in the flashlight and also helps to improve thelife span of the bulb.

[0074] In a further embodiment the switch can provide for flashlightoperation at a regulated voltage, for example a four cell flashlight(6V) may be regulated to work at 4V with a suitable bulb for the 4Voutput. If the user wants to have brighter light for a short period, theunregulated voltage can be applied to the bulb or, if the voltage hasdropped, a step-up can be performed to yield a very bright light.

[0075] The invention is frequently described herein with reference tothe operation of a light bulb. It is to be understood that this is onlyby way of example and that the invention can be used with any other loadeg. a heater, motor or any other electrical device.

[0076]FIG. 6 of the accompanying drawings illustrates a microchip 110according to another form of the invention which is supplied withelectrical power from a source 112. The microchip includes a powercontrol switch 114, a timer 116 and a control circuit 118. A load 120 isconnected to the microchip. The switch 114, under the control of thecontrol unit 118, controls the connection of the power source 112 to theload 120 in accordance with various criteria.

[0077] A switch 122 is used to turn the microchip on or off. This switchgives to the user overall control of the microchip so that the microchipcan be enabled or disabled.

[0078] An optional second switch 124 is used as a mode selector switch.This enables a user to select different modes or functions of themicrochip so that the load 120 is controlled, or caused to operate, in adifferent way.

[0079] The drawing illustrates a third switch 126 which is referred toas an activity detector switch. Although shown as a switch this could,in some situations, be a symbolic notation only for activity of the usercould be detected in any appropriate way by making use of any suitablemonitoring system.

[0080] The switch 126, which is used to detect activity, can be one of anumber of switches which are used to control operation of the controlunit 118. Alternatively the switch 126 can be operated in a particularway or sequence to provide different command signals to the control unit118 so that the control unit can function according to requirement.

[0081] Assume for example that the load 120 is an animated toy orsimilar device which is powered by the power source 112 in a mannerwhich is determined by the operation of the switch 126 and, whereappropriate, by the switch 124. Each time the switch 126 is activatedthe timer 116 is reset. Thus the switch 114 will remain closed and theload 120 will be energised while there is activity at the switch 126.Once the activity ceases the timer 116 starts its timing period. If,during this period, there is no activity at the switch 126 then thetimer 116 will complete its timing period and, at the end thereof, willcause the switch 114 to open circuit. In other words the delayedswitch-off function will be implemented a predetermined period, which isdetermined by the timer interval, after the last activity is detected bythe control unit 118.

[0082] Any appropriate means may be used for detecting activity relatingto the microchip. As stated, the switch 126 may be operated in a varietyof different ways in order to exert different control functions or,alternatively, the switch 126 may be one of a plurality of similarswitches each of which exerts a respective control function. Dependingon the protocol a switch which is opened or closed, can be detected asactivity. Thus the timer could be reset by a high input signal, a lowinput signal or a transition signal in a chosen direction eg.low-to-high, or high-to-low, or any combination thereof.

[0083]FIG. 7 of the accompanying drawings illustrates a circuit 210 forcontrolling the supply of power from a battery power source 212 to aload 214. The nature of the load may vary from case to case. In thefollowing description the load 214 is described as being an electriclight bulb but this is given merely by way of a non-limiting example.

[0084] A number of signal switches 216 are connected to the circuit andare used for controlling its operation. A number of light emittingdiodes 218 are connected to the circuit 210 and are used for indicatingcertain aspects of the operation of the circuit and for providing anindication of the status of the battery 212, in the manner which isdescribed hereinafter.

[0085]FIG. 8 is a more detailed block representation of the circuit 210.The circuit 210 is an integrated circuit and includes a control unit220, an analogue to digital converter 222, an input and output unit 224which functions as an interface, timing, reset and oscillator modules226, 228 and 230 respectively, a current switch 232 and a memory module234.

[0086] The switches 216 are used for actuating the control unit 220through the medium of the input and output unit 224 Selected informationgenerated by the control unit 220 is transferred to the light emittingdiodes 218 through the medium of the input and output unit 224.

[0087] The voltage from the power source or battery 212 is measured andconverted to a digital format by the analogue to digital converter 222.The converter also detects whether the current switch 232 is opened orclosed when the battery voltage is measured and, in this way, is capableof providing a digital measurement of the open circuit voltage or theclosed circuit voltage of the battery 212.

[0088] Once the input voltage has been digitally measured its value,referred to as V_(D) herein, is transferred to the control unit 220.

[0089] Pre-programmed reference voltages are stored in the memory module234. The number of reference voltages which are stored is determined bythe number of categories in which the battery voltage can be classified.For example a three category indication of good, medium and bad requiresat least two reference values V_(G) and V_(M). The battery indicationsare as follows depending on the relative voltage levels:

[0090] V_(D)≧V_(G)—“good” indication;

[0091] V_(G)>V_(D)≧V_(M)—“medium” indication; and

[0092] V_(D)<V_(M)—“bad” indication.

[0093] By actuating a selected switch 216 the control unit 220 is causedto operate, substantially in the manner which is described in thespecification of international application No. PCT/ZA99/00107, and theswitch 232 is closed thereby to connect the battery 212 to the load 214.As indicated the battery voltage is measured and compared to thereference voltages stored in the memory module 234. An indication isthen substantially immediately given of the status of the battery viathe appropriate light emitting diode or diodes 218 which are activatedin a predetermined manner.

[0094] The measurement of the closed circuit voltage of the battery canbe stored in the memory module 234. When the battery is disconnectedfrom the load the open circuit battery voltage can be measuredautomatically by the action of the control unit, at regular intervals,and compared to the closed circuit voltage stored in the memory module234. If the open circuit voltage drops below the stored value of theclosed circuit voltage the category (good, medium, bad) in which thebattery is classified can be altered to reflect the change in thebattery condition.

[0095] The timing unit 226 is used to control the intervals at which thebattery voltage is measured and for controlling the frequency at whichthe light emitting diodes 218 are pulsed. It is to be noted that whenthe switch 232 is open at least one of the diodes 218 may be pulsed at avery low duty cycle, which is controlled by tine timing unit 226, toprovide a find-in-the-dark facility. The low duty cycle ensures that thepower consumption of the flashing LED is kept to a low value.

[0096] The control unit is capable of modulating the power supply, underappropriate conditions, to ensure that the power which is supplied tothe load 214 is kept substantially constant as the battery voltage dropsdue to power consumption. Assume for example, as is shown in FIG. 11,that the voltage of the battery 212 is initially at 8V and that the load214 is a 3.6V bulb. As the battery voltage drops due to the charge inthe battery being diminished through usage the duty cycle of the powersupplied to the load increases. In the range of from 8V down to 3.6V,the duty cycle is non-linear with respect to voltage and, once thevoltage crops to 3.6V, the duty cycle is 100%.

[0097] In the case of a light bulb it is important to note that thelight bulb will give substantially constant illumination if its averagepower consumption is constant. FIG. 9 illustrates a way in which thebattery voltage can be modulated or switched by the oscillator 230acting directly or indirectly on the current switch 232. In thisinstance, with the battery voltage at 6V, pulses with a duration ofapproximately 0.36 ms are generated at a frequency of approximately 1kHz which is controlled by the oscillator unit 230.

[0098] Apart from errors based on resolution it is only after thebattery voltage drops below 3.6V that the light bulb illumination startsto degrade. The acceptable margin for error determines the accuracyrequired for measuring the voltage across the battery as well as theresolution with which the duty cycle can be constructed.

[0099]FIG. 12 shows a graph of the error levels caused by digitalrounding if the battery voltage (V_(D)) is measured to an accuracy of0.4V and a 5% resolution is used for the duty cycle construction.Clearly this accuracy is a function of complexity and cost, and can beincreased within practical limits.

[0100] The functions of battery status and constant illumination requirea voltage measurement of the closed circuit voltage of the battery ie.when the load is connected to the battery. From a cost andimplementation perspective these functions are logically provided by theintegrated circuit 210. On the other hand if the product with which thecircuit is used, eg. a flashlight, has a find-in-the-dark feature, eg.an LED which flashes at a low duty cycle, then it stands to reason thatthe battery life indication can be provided by the find-in-the-dark(indicator to reduce the component count.

[0101] The effect of constant illumination can also be achieved bystepping up the input voltage to a higher voltage level. This can beimplemented by using various DC-to-DC step-up converter techniques whichare known in the art. The advantage of a step-up approach means thathigher illumination levels are achievable than what would otherwise bethe case when the battery voltage drops. This however would be at thecost of additional components. One can also make use of a step downconverter to achieve substantially constant illumination.

[0102] The switching rate of the switch, ie. the modulation rate of thepower supplied to the load, must be such as to avoid overheating duringthe on part of the cycle and to prevent flickering occurring which isvisible to the human eye. On the other hand with every switching actionsome power losses occur. Thus lower modulation frequencies are moreefficient and components are more feasible. For Xenon bulbs a 1 kHzswitching rate appears to be a good compromise.

[0103]FIG. 10 is a simple flow diagram representation for the on/offfunction of a flashlight with a continuous indication of battery status.

[0104] When the batteries are inserted into the flashlight (300) flagsin the control unit are set (310). At this stage the circuit 210 doesnot have information about the battery status and the open circuitvoltage of the battery must be measured. Optionally the flashlight ismomentarily turned on to measure the closed circuit voltage. Thisenables the battery life indicator flag (BLI flag) to be set inaccordance with the measured voltage.

[0105] A block 315 indicates that the load is switched off at this time.

[0106] If a find-in-the-dark (FID) function is implemented theintegrated circuit 210 causes the corresponding LED (good, medium orbad) to flash even though the bulb 214 is not illuminated (325).

[0107] Upon user activation (320) the inputs 216 to the circuit 210become active. Within normal state of the art practices like debouncingetc. the control unit 220 turns the current switch 232 on. In order toprotect against overheating the calculation of the correct duty cyclefor the applicable input battery voltage must be made faster than thelowest duty cycle, or the duty cycle must be set to its lowest (safest)level (330), so that the duty cycle calculations can be done. Thereafterthe correct duty cycle, which determines the illumination of the bulb214, can be selected (335).

[0108] Once the closed circuit voltage has been measured and convertedthe resulting digital value V_(D) can be compared against referencevalues in order to categorize and indicate the status of the battery(340) (BLI=battery life indicator).

[0109] The control unit 220 performs the functions up to step 345 in ashort time. A check is constantly required (step 345) to see if the userhas released the appropriate input switch 216. If not the steps 335, 340and 345 are repeated. The input switch 216 is monitored more activelythan the voltage levels in order to conserve power. Once the release ofthe input switches has been detected (345) the control unit 220 proceedsto check the voltage levels to adjust for a possible drop in supplyvoltage (350) and to check the input switches 216 (355) for a terminatecommand. If a terminate command is received the control unit 220 turnsthe current switch 232 off (block 360).

[0110] Upon the subsequent release of the input switch 216 by the userthe control unit 220 again checks the input switches (365) for anactivation command and performs the find-in-the-dark (FID) functionusing the battery life indicator (BLI) category determined during theprevious on state. The control unit 220 also monitors the batteryvoltage level to determine if the open circuit voltage has dropped lowerthan the previously measured closed circuit voltage. As has been statedthis may require a change in the indicated category of the batterystatus.

[0111]FIG. 13 illustrates a further aspect of the invention. In thiscase use is made of a control circuit or microchip 400 substantially ofthe kind described hereinbefore which is supplied with power from abattery 402. The control circuit has three signal switches 404, 406 and408 respectively connected to respective inputs 404A, 406A and 408A. Anoutput from the control circuit controls a current switch 410 which isused to connect power from the battery 402 to a load 412.

[0112] In this example of the invention the inputs 404A and 406A arereferred to as main and mode inputs respectively and are edge triggeredinputs. In other words these inputs are designed to work withnon-latching input devices 404 and 406. As such, a first momentaryactivation of the respective signal switch 404 or 406, as the case maybe, causes the control circuit 400 to interpret the input as an “on”command and a second activation will be interpreted as an “off” command.

[0113] By way of contrast the input 408 is a state triggered input. Thusa high input level is interpreted as an “on” command while a low inputlevel is interpreted as an “off” command.

[0114] It is to be borne in mind that the levels referred to are merelyby way of example and that the “on” and “off” commands can result frominput signals at levels which are opposite to or different from what hasbeen described.

[0115] Assume for example that the signal switch 408 is connected via asignal wire to the input 408A. The signal switch 408 may for example bea controller, such as a door-operated switch, in a vehicle whichcontrols illumination inside the vehicle. For example when a door of thevehicle is opened the load 412, which is a lamp in the vehicle, isenergised by the battery 402. In this configuration the switch 404 isinstalled, for example, at the point of illumination ie. near to theload 412.

[0116] The aforementioned configuration eliminates the requirement for acurrent carrying lead or wire from the signal switch 408 to the load 412ie. a wire which can carry the full load current drawn by the lamp 412.This reduces the weight and cost of the lighting installation.

[0117] It is important that a signal from the switch 404 to the input404A can override an input from the signal switch 408 to the input 408A,but not vice versa. Thus with the light 412 on due to a high inputprovided by the signal switch 408 an activation of the switch 404 willturn the light off. However if the light is on due to a command whichhas previously been input from the switch 404 to the input 404A a highlevel resulting at the input 408A by activation of the signal switch 408will not affect the light condition and neither will a subsequent lowinput at the input terminal 408A.

[0118] The control circuit 400 will turn the light 412 off after an onperiod of a defined duration, for example 30 minutes or an hour,depending on requirement, irrespective of which input caused the oncondition.

[0119] The mode input 406A exhibits the same protocol or hierarchy asdoes the main input 404A with regards to the input 408A. Thus a signalinput via the switch 406 will override a state previously determined byan input from the switch 408, but not vice versa. A signal applied tothe mode input can be used to reduce the power to the light by causingthe duty cycle to be reduced and this will result in the light dimming.

[0120] A further aspect of this invention is shown in FIG. 14 whichdepicts, somewhat schematically, a housing 500 of a conventionalelectromechanical switch 502 operated via a button 504 The microchip orcontrol circuit 12, eg. of the type shown in FIG. 2, is packaged insidethe housing 500. The switch 502 acts as a signal switch 18 and controlsthe connection of a power supply 506 to a load 508. In other respectsthe circuit acts in the manner described hereinbefore. The switch 502may vary according to requirement and for example may be a pushbuttonswitch (push-to-make release-to-break; eg. Carling switch P27L13L), atoggle switch (single pole double throw; eg. Arcolectric V1722RO) or arocker type switch (eg. Arcolectric PLC1522AA) Using the two wire modethe switch can be pin compatible albeit polarity sensitive, but canprovide intelligent decision making actions.

1. A switching circuit for controlling the supply of power from anexhaustible power source to a load which includes a control circuit, apower switch which is controlled by the control circuit or aman-machine-interface (MMI) and which, when closed, connects the powersource to the load, and at least one signal switch input to the controlcircuit which can detect if a switch is open or closed and does not forma serial part of an energy transfer circuit from the power supply to theload and wherein the control circuit further provides at least onefunction selected from the following functions: (a) a power source levelindication which is determined by comparing a fixed reference voltage toa closed circuit supply voltage and which is displayed to a user, evenat times when the load is disconnected from the power source; and (b) afunction wherein the control circuit compares the voltage of the powersource to a fixed reference voltage and, in response to the comparison,controls a duty cycle of the power switch to provide a substantiallyconstant supply of power from the power source to the load over avoltage range of the power source.
 2. A switching circuit according toclaim 1(b) wherein the level of said substantially constant supply ofpower is determined in accordance with a level selected by means of asignal at the signal switch input.
 3. A switching circuit according toclaim 1 or 2 wherein the control circuit provides at least one furtherfunction in response to a sequence of activation signals at the signalswitch input.
 4. A switching circuit according to claim 3 wherein thesequence of activation signals is changed dynamically to select off onthe next activation or deactivation signal once any specific selectionfrom the functions has been active for a predetermined period of time.5. A switching circuit according to any one of claims 1 to 4 wherein thecontrol circuit monitors the voltage of the power source and when thepower source voltage drops below an operating voltage of at least oneelement of the control circuit, the power switch is controlled directlyby a signal at the signal switch input to connect the power source toload.
 6. A switching circuit according to any one of claims 1 to 5wherein the power source level indicator also functions as afind-in-the-dark indicator.
 7. A switching circuit according to any oneof claims 1 to 6 wherein the control circuit monitors the voltage of thepower source at predetermined intervals in the “on” and in the “off”state of the power switch.
 8. A switching circuit according to claim 7wherein a load is automatically connected to the power source before thevoltage of the power source is measured.
 9. A switching circuitaccording to any one of claims 1 to 8 wherein the power source levelindication is additionally based on a measurement of the level of thesupply voltage when the power switch is not conducting.
 10. A switchingcircuit according to any one of claims 1 to 9 wherein the load is anelectric motor and the duty cycle of the power switch is controlled toachieve a substantially constant operation of the motor over a range ofpower source levels.
 11. A switching circuit according to any one ofclaims 1 to 9 wherein the load is a lighting element and the duty cycleof the power switch is controlled to achieve a substantially constantillumination level of the lighting element over a range of power sourcelevels.
 12. A switching circuit according to any one of claims 1 to 11wherein the power source level indication additionally gives anindication of a selected function or made of the control circuit.
 13. Aswitching circuit according to any one of claims 1 to 12 which ispackaged within the housing of a conventional electromechanical switch.14. A switching circuit according to any one of claims 1 to 13 whereinthe substantially constant power supplied to the load is achieved bystepping up the input voltage to a higher voltage for the load.